Biomass is generally understood to refer to the amount of a living biological organism (“bio”) present in a given space at a particular point of time, as expressed as the amount of substance (“mass”). Biomass may be expressed as “seibutsutairyou” or “seibutsuryou” in Japanese, and may be called “standing crop” in ecology. Biomass is commonly quantified by mass or amount of energy, and may be expressed as the dry weight of a living biological organism per unit area. In many cases, plants are used to create biomass, and increasing plant biomass is considered useful not only for providing biofuels or renewable energies but also for stabilizing food supply through increased crop yields.
As for increase in plant biomass, various techniques have conventionally been adopted to develop a new, industrially useful plant variety, such as a hybridization breeding method in which different plants are crossed to select a superior progeny, and a mutation breeding method which induces mutation in a plant. In recent years, there have been developed genetically modified plants in which a useful gene is introduced to exhibit its function. For the purpose of developing such a new plant variety, a method for assembling genes that can impart superior characters is effective, but under the circumstances where further improvement in crop productivity is needed, only limited types of genes are available at present, and there is a desire to identify genes involved in increased biomass, particularly in high-yielding character.
Promotion of plant growth is in many cases influenced by plant hormones, and brassinosteroids are known examples of the plant hormones. Brassinosteroids are a group of compounds having a steroid backbone and typified by brassinolide. Brassinosteroids have the following activities related to plant growth: (i) promotion of the elongation and growth of stems, leaves and roots; (ii) promotion of cell division; (iii) promotion of differentiation of mesophyll cells into ducts or tracheids; (iv) promotion of ethylene synthesis; (v) promotion of seed germination; and (vi) imparting of resistance to environmental stress. There has been an attempt to increase the yield of Oryza sativa by utilizing the aforementioned physiological activities of brassinosteroids and introducing a brassinosteroid biosynthesis gene into O. sativa (Non-Patent Literature 1). However, the effects of this attempt are not necessarily satisfactory. Hitherto, a certain number of genes involved in the synthesis and signaling of brassinosteroids have been identified, but no gene has been identified which can fully achieve increased plant biomass.